Motion analysis is an important part of the discipline of biomechanics, and can be associated with various applications such as, for example, balance assessment, force sensing measurement, sports science training, physio analysis, and fitness equipment operation, etc. Motion analysis is typically performed based on images, in which a system captures a sequence of images of a subject (e.g., a human being) when the subject is engaged in a specific motion. The system can then determine, based on the sequence of images, the positions of various body segments of the subject at a given time. Based on the positions information, the system can then determine a motion and/or a posture of the subject at that time.
Current technologies provide various ways of performing image-based motion analysis. One approach is by tracking a motion of markers emitted by the subject. For example, the subject can wear a garment that includes a number of markers. The markers can be passive reflector (e.g., with VICON™ system) or active emitter of visible light or infra-red light (e.g., with PhaseSpace™ system). The system can then use a plurality of cameras to capture, from different angles, a sequence of images of the markers when the subject is in a motion. Based on the sequence of images, as well as the relative positions between each camera and the subject, the system can determine the motion of the subject by tracking the motion of the markers as reflected by images of the markers included in the sequence of images.
Another approach is by projecting a pattern of markers on the subject and then tracking the subject's motion based on images of the reflected patterns. For example, Microsoft's Kinect™ system projects an infra-red pattern on a subject and obtains a sequence of images of the reflected infra-red patterns from the subject. Based on the images of the reflected infra-red patterns, the system then generates depth images of the subject. The system can then map a portion of the depth images of the subject to one or more body parts of the subject, and then track a motion of the depth images portions mapped to the body parts within the sequence of images. Based on the tracked motion of these depth images portions (and the associated body parts), the system can then determine a motion of the subject.
There are disadvantages for both approaches. With the VICON™ system, the subject will be required to wear a garment of light emitters, and multiple cameras may be required to track a motion of the markers in a three-dimensional space. The additional hardware requirements substantially limit the locations and applications for which the VICON™ system is deployed. For example, the VICON™ system is typically not suitable for use at home or in an environment with limited space.
On the other hand, the Kinect™ system has a much lower hardware requirement (e.g., only an infra-red emitter and a depth camera), and is suitable for use in an environment with limited space (e.g., at home). The accuracy of the motion analysis performed by the Kinect™ system, however, is typically limited, and is not suitable for applications that demand high accuracy of motion analysis.